The present invention relates to a scanning clock generating device for an optical scanner.
As well known in the art, an optical scanner is adapted to scan a desired surface by a light beam to write optical information or read out image information. In this type of scanner, a light beam is usually deflected at a constant angular velocity by a polygon mirror, a holoscanner or like rotary optical deflector. The scanning velocity on the selected surface is maintained constant typically by an F-theta lens. However, because an F-theta lens is expensive due to its specially designed characteristics, eliminating the need for such a lens would be desirable. Meanwhile, a polygon mirror which causes a light beam to scan at various angular velocities has recently been proposed (Japanese Patent Application No. 59-274324). An F-theta lens cannot be used with the variable angular velocity type polygon mirror because it would fail to maintain the scanning velocity constant.
An optical scanner uses a scanning clock in order to turn on and off a light beam during the course of scanning. Assuming that a period of time assigned to reading or writing on one pixel of information is T, the frequency f.sub.k of the scanning clock is represented by 1/T. Where an F-theta lens is not used and, therefore, the scanning velocity on a surface to be scanned is not constant, holding the frequency f.sub.k of the scanning clock unchanged would introduce distortions into the information writing and reading operations.
It follows that if the clock frequency f.sub.k were variable pixel by pixel, optical scanning free from distortions could be realized without resorting to an F-theta lens. However, varying the clock frequency f.sub.k on a pixel basis is not always easy. In light of this, there has been proposed a method which divides a predetermined scanning range into a plurality of blocks and discontinuously varies the clock frequency block by block, while assigning a particular frequency to each of the blocks (Japanese Unexamined Patent Publication No. 58-87965). The problem with such a method is that because the frequency is discontinuously varied from one block to another, in the case of writing optical information, for example, the discontinuous frequencies are reflected by a kind of discontinuity in recorded images.